# Check Persistence

Checks can persist information between runs. This can for example be useful for monitoring relative increases of a metric instead of absolute thresholds.

Persistence in checks is based on the file-system. Write all files that you want to persist to the $CHECKSON_DIR/persistent folder. When the next run is executed, the $CHECKSON_DIR/persistent will restored with the contents that the current run wrote to it.

CHECKSON_DIR is an environment variable that is set by the Checkson cloud executor when it runs your Docker image. You will have to create the persistent subfolder before you create files in there.

Let's look at an example:

import sys
import os

checkson_dir = os.environ.get('CHECKSON_DIR', '/tmp')
persistent_dir = f"{checkson_dir}/persistent"
result_file = f"{persistent_dir}/result"


def write_result(result):
    if not os.path.exists(persistent_dir):
        print(f"Creating {persistent_dir}")
        os.makedirs(persistent_dir)

    print(f"Persisting result '{result}' to {result_file}")
    with open(result_file, "w") as f:
        f.write(str(result))


def read_last_result():
    if not os.path.exists(result_file):
        print(f"No result file found: {result_file}")
        return False
    with open(result_file) as f:
        last_result = f.read().strip()
        print(f"Persisted last result: {last_result}")
        return "True" == last_result


def main():
    last_result = read_last_result()
    result = not last_result
    write_result(result)

    print(f"Last result: {last_result}, new result: {result}")

    if result:
        print("Check successful")
        sys.exit(0)
    else:
        print("Check unsuccessful")
        sys.exit(1)


if __name__ == "__main__":
    main()

This check will alternate between successful and unsuccessful runs. It does that by writing the result into the file $CHECKSON_DIR/persistent/result. Before writing the file, it creates the folder $CHECKSON_DIR/persistent.

If the environment $CHECKSON_DIR is not set, the example uses /tmp as a fallback. This is useful for testing the script locally, so you do not have to set the environment variable.

The size of the files in $CHECKSON_DIR/persistent should not exceed 50 KiB.

The complete example can be found on GitHub.